1. Field of the Invention
The present invention relates to a beam splitting prism, a method of manufacturing the beam splitting prism, and an all-optical switching device, particularly to the all-optical switching device applicable to DEMUX (Demultiplexer) which extracts a specific time component in a time-division multiplexing signal, the beam splitting prism suitable for an optical component of the all-optical switching device, and the method of manufacturing the beam splitting prism.
2. Description of the Related Art
In recent years, the amount of information by communication is increased due to wide spread of information and communication network such as the Internet, and a high-capacity communication system which can transmit the huge amount of information at high speed is demanded. An optical communication system receives attention as the high-speed transmission technology from points of view of extremely short-pulse characteristics of light, broadband characteristics of an optical fiber used for the transmission, and capability of long-distance transmission. Particularly in a time division multiplexing mode of communication, DEMUX plays an important role in separating and extracting a specific time interval component from multiplexed signal pulses which are transmitted through the optical fiber.
In the optical communication system currently used an optical switch which is electrically controlled is used as an optical switching element extracting the specific time interval component from the signal pulses. However, when the high-capacity communication further proceeds in the future, for example an optical switch which optically turns on and off the signal light by an optical pulse is required in order to realize extremely high-speed communication not lower than tera-bit/s (Tbit/s) as unit corresponding to the increasing amount of information. For such an optical switch controlled by the light, the optical switch utilizing a nonlinear optical medium exhibiting a change in absorption or a change in refractive index caused by the irradiation of light is mainly researched.
The inventors have realized the element in which an extremely high-speed time-division multiplexing signal pulse in an order of Tbit/s is split (de-multiplexed) into signal pulses in a lump with an absorption change type two-dimensional optical switch (see Japanese Patent Application Laid-Open (JP-A) No. 11-15031). The lump change type optical switch will become the needed optical switch for a future system in which the extremely high-speed optical communication system (Tbit/s), which can be controlled by the light, is combined with a medium-speed optical communication system (up to 40 Gbit/s), which can be control by electricity. Further, the inventors have realized a Kerr effect type optical switch utilizing induced birefringence caused by the irradiation of the nonlinear optical medium with the light, and dramatically improved an on-off ratio in the optical switching (Japanese Patent Application Laid-Open (JP-A) No. 2002-062558).
Further, the inventors have realized the, optical switch stably operating irrespective of polarization state in such a manner that the optical switching is performed by separating the light beam into the polarized lights orthogonal to each other and optical components such as a beam splitting element and a lens array are configured by precisely forming and bonding the components at a surface orthogonal to a propagating direction of the light (Japanese Patent Application Laid-Open (JP-A) No. 2003-149693).
However, in the above-mentioned contact type optical switch, there is a problem that a configuration of the beam splitting prism used as the optical component is complicated and it is difficult to secure spatial and temporal superimposition in condensing spots of a conrtol pulse and a signal pulse during condensing the light beams at multiple points.
FIGS. 17A and 17B show a configuration of the conventional beam splitting prism. In the beam splitting prism shown in FIG. 17A, a total reflection mirror 6C for adjusting an angle is arranged in the propagating direction of a parallel beam input from an incident end face 7 into an optical element 4, and the input parallel beam is reflected with the total reflection mirror 6C and split with a half mirror 5 into two beams which respectively form the same angle to the half mirror 5. In the propagating direction of one of the two split beams, a total reflection mirror 6A is provided while being parallel to the half mirror 5, and the other beam is made to be parallel to the beam in such a manner that the other beam is reflected with the total reflection mirror 6A.
In the propagating direction of each of the parallel beams, total reflection mirrors 6B are respectively arranged in parallel to each other at positions where optical path lengths of the parallel beams after being split with the half mirror 5 are equal to each other. After each of the parallel beams is reflected with the corresponding total reflection mirror 5B, each of the parallel beams is output from an outgoing end face 8 which is formed so as to be orthogonal to an optical axis of the parallel beam after the reflection. That is to say, while the total reflection mirror 6B causes the optical path lengths of parallel beams split with the half mirror 5 to be equal to each other, the total reflection mirror 6B has a function of adjusting an outgoing angle. Since each of the output beams has the same optical path length from the split at the half mirror 5 to the incidence to the total reflection mirror 6B, the two split beams propagate at the same time to a plane normal to the propagating direction.
Using three kinds of half mirrors 5 as shown in FIG. 17B, second step half mirrors 5B are respectively arranged in the propagating direction of each beam which has been split into two beams with a first (first step) half mirror 5A, and the beam which has been split into the two beams at the first step can be split into four beams by further splitting the beam into two beams at the second step.
As shown in FIGS. 17A and 17B, in these beam splitting prisms, the number of components to be bonded is large and the number of surfaces to be bonded in assembly is also large. There are at least three bonding surfaces for both the two-split prism and the four-split prism.
In almost all the components to be bonded, the incident end face is not parallel to or orthogonal to the outgoing end face, so that an outgoing direction is changed when a bonding angle is deviated. Further, the outgoing angle is changed when rotational deviation is generated in a bonding surface during bonding the components, and an outgoing position is shifted when a bonding position is deviated. As a result, it is difficult to maintain parallelism and positional accuracy of the split beams. That is to say, in the optical component in which the incident end face is not parallel to or orthogonal to the outgoing end face like the prism, it is not easy to polish the optical component with high accuracy, and it is difficult to secure the accuracy in the outgoing direction when the multiple optical components are bonded to one another.
Further, in the two-split prism shown in FIG. 17A, any split beam does not have coaxial relation with the incident light beam, and it is impossible to insert the prism in an aligned optical system.
Similarly to Japanese Patent Application Laid-Open (JP-A) No. 2002-328005, in the case where the prism which splits the incident beam and outputs the split beams from a plurality of positions is formed to obtain the plurality of split beams propagating through the predetermined optical path lengths within the prism in such a manner that the transparent mediums are laminated in one direction and the splitting optical element or the reflecting surface is formed in the bonded surfaces of the laminated transparent mediums, there is a problem that the beams can be outputted only at the outgoing positions of respective split beams on the outgoing end face, the outgoing positions having, in one direction parallel to the outgoing end face, an interval equal to a difference of optical path lengths.
In this case, in a case in which, as the splitting optical system, another optical element is inserted in the optical path in order to adjust the outgoing position, changing of the outgoing position caused by changing of positions of the prism and the optical element due to temperature variation. As a result, it is difficult to use stably as the splitting optical system.